Conventionally, for producing the aforementioned pattern for casting (hereinafter referred to as "casting pattern"), the configuration of the pattern is defined as three dimensional CAD (computer aided design) system data. The data is then divided into a plurality of parts, each representing a division block with an appropriate thickness. This allows the entire pattern to be machined with ease. Specifically, as shown in FIGS. 1A through 1D, each casting pattern is worked from a block A (FIG. 1A) by a numerical control machine (hereinafter referred to as "NC machine") in such a way that the casting pattern, as divided into a plurality of parts (hereinafter referred to as "division block"), will be subject to an independent machining process, e.g. milling process. For the machining work, it is necessary to create a program with numerical control data (hereinafter referred to as "NC data") which establishes the numerical control of the NC machine. The program is created, as stated above, base upon three-dimensional data of the division block Aa. In the illustrated examples, the machining proceeds along a contour of each division block Aa, in accordance with the program, to form a pocket in the block. Generally, the division block Aa of the casting pattern is made of expanded polystyrene or of styrol resin.
Next, as shown in FIG. 1B, the division block Aa is fixed on a block fixing surface 50 of a working table of the NC machine via a jig (not shown). According to the programmed NC data, a front surface of the division block Aa is processed or profiled by a cutting tool 51 having a protruding blade length S+.alpha. which is slightly longer than the thickness S of the division block Aa and a blade length S+.beta. which is slightly shorter than the protruding blade length S+.alpha. (blade length has to be longer than the block thickness).
Then, after the processing of the front surface is finished, the division block Aa is turned over on the block fixing surface 50 and it is fixed again to the block fixing surface 50 of the working table of the NC machine. Then, as shown in FIG. 1D, like the front surface, the rear or reverse surface of the division block Aa is cut, based on the programmed NC data, to obtain a predetermined shape. By combining or assembling the plural division blocks Aa, each having predetermined shape which is produced in a similar manner, the casting pattern is created.
However, because there is insufficient stiffness around the through hole portion B, processing of the front surface is difficult. Specifically, when the division block is turned over and processing of the rear surface begins, the position of the milling tool during processing of the through hole on the rear surface deviates from that of processing of the through hole on the front surface so that a high dimensional accuracy of the through hole cannot be assured which might lead to a formation of steps within the hole.
Further, if the number of the division blocks Aa is decreased and the thickness of each division block Aa increased, the blade length of the cutting tool 51 will increase and will not correspond to high speed processing requirements.
Further, because pockets are produced on the block and the processing is executed along the contour of the division block Aa, a large number of steps are required to make or to produce the program on a numerical control tape (hereinafter referred to as "NC tape"). Moreover, as shown in FIG. 1C, when a thin rib C is to be formed, high dimensional accuracy cannot be assured because of vibration which occurs during the machining process.
These problems in the conventional production method of a casting pattern have been left unsolved up to now.